1. Field of the Invention
The present invention relates to a lubricant oil composition. More particularly, the present invention is concerned with a lubricant oil composition for use in a refrigeration system, specifically a refrigeration system in which, from the viewpoint of the environmental protection, substitute refrigerants are used for chlorine-containing refrigerants, such as CFC-12 (1,1-dichloro-1,1-difluoromethane) and HCFC-22 (chlorodifluoromethane), which are likely to destroy the ozone layer. Examples of substitute refrigerants include: (1) fluoroalkanes, particularly lower hydrocarbons having 1 to 5 carbon atoms, such as HFC-134a (1,1,1,2-tetrafluoroethane), HFC-134 (1,1,2,2-tetrafluoroethane), HFC-152a (1,1-difluoroethane), HFC-32 (difluoromethane), HFC-125 (pentafluoroethane), HFC-143a (1,1,1-trifluoroethane) and HFC-227ea (1,1,1,2,3,3,3-heptafluoropropane), which do not damage the ozone layer; (2) hydrofluoroethers (HFE) (hereinafter frequently referred to simply as "HFE"), which do not damage the ozone layer and, in addition, are expected to lower the GWP (Global Warming Potential); (3) mixtures of the refrigerants mentioned under (1) and (2) above.
2. Discussion Of Related Art
CFC-12 is still currently, widely used as a refrigerant in refrigeration equipment for car air conditioners and refrigerators, while HCFC-22 is widely used as a refrigerant in refrigeration equipment for room air conditioners. However, development of a refrigerant which can be used in substitution for chlorine-containing refrigerants, such as CFC-12 and HCFC-22, has been desired from the viewpoint of the protection of the ozone layer.
As substitute refrigerants, the above-mentioned lower hydrofluorocarbons having 1 to 5 carbon atoms are promising. Of these, hydrofluorocarbons having 1 to 2 carbon atoms are particularly preferred.
In conventional refrigeration systems using CFC-12 or HCFC-22, mineral oil or alkylbenzene is used as a lubricant for a compressor of the refrigeration system. CFC-12 and HCFC-22 exhibit high oleophilicity since they contain chlorine atoms, so that they are miscible with mineral oil and alkylbenzene over a wide temperature range. Accordingly, even in the refrigeration systems where evaporation and condensation of the refrigerant are repeated, phase separation of CFC-12 or HCFC-22 as the refrigerant from the lubricant does not occur.
However, various types of hydrofluorocarbon refrigerants (hereinafter frequently referred to simply as "HFC refrigerants") and HFE refrigerants are not satisfactorily miscible with hydrocarbon compounds, such as mineral oil and alkylbenzene, since such refrigerants contain no chlorine atom. Therefore, when a hydrocarbon compound, such as mineral oil or alkylbenzene, is used as a lubricant, various serious problems occur. For example, because the lubricant is replaced by the refrigerant in a compressor, the lubrication becomes unsatisfactory. And the lubricant adheres to the inner wall of the heat exchanger, leading to a lowering of the heat exchange efficiency.
A lubricant for use in refrigeration equipment employing, as a refrigerant, a hydrofluorocarbon (HFC) such as HFC-134a, and a hydrofluoroether (HFE), should be miscible with either HFC or HFE at least over a temperature range from 0.degree. C. to 50.degree. C., preferably over a range from -20.degree. C. to 70.degree. C., more preferably over a range from -40.degree. C. to 90.degree. C., and most preferably over a still wider temperature range.
Various polyalkylene glycol compounds, polyol ester compounds and polycarbonate compounds have been proposed as lubricants highly miscible with hydrofluorocarbons, such as HFC-134a, for use in combination with such hydrofluorocarbons. For example, polyalkylene glycols having at least two hydroxyl groups (especially, polyoxypropylene glycol) disclosed in the specification of U.S. Pat. No.4,755,316, are described as exhibiting a good miscibility with hydrofluorocarbons, such as HFC-134a, over a wide temperature range. However, the temperature range over which polyalkylene glycols are miscible with HFC-134a is still unsatisfactory for their functioning as lubricants, and improvement of the miscibility, especially at high temperatures, is required.
Polyoxalkylene glycols have not only unsatisfactory lubricating properties under application conditions, but also high moisture absorption (or water absorption) properties. Therefore, various problems are likely to arise, such as corrosion of metals, and lowering of a volume resistivity (a lowering of a volume resistivity causes a problem in the case of a closed type refrigeration equipment, such as a refrigerator.) Accordingly, polyoxyalkylene glycols are not good lubricants for a refrigeration system from a practical point of view.
Polyol ester compounds, disclosed in Japanese Patent Application Laid-Open Specification Nos. 3-128991 and 3-179091 and polycarbonate compounds in Nos.5-32688 and 5-86391, are generally considered to be also highly miscible with HFC-134a. However, since such compounds exhibit high moisture absorption properties due to the polar groups contained therein, a problem in durability in terms of hydrolysis resistance is caused.
In addition to the above-mentioned polyalkylene glycol and polyol ester oils, fluorine-containing oils are considered to be lubricants miscible with fluoroalkanes, such as HFC-134a.
With respect to the fluorine-containing oils, the following patent application publications are noted.
In Japanese Patent Application Laid-Open Specification No. 60-96684, it is taught that when a fluorolubricant oil, such as a fluorinated silicone or a perfluoropolyether, is used in a fluorocarbon working medium for a heat pump or the like, the heat resistance of the fluorocarbon working medium is improved. However, no description is made therein with respect to the miscibility of the lubricant oil with a hydrofluoroalkane to be used as a working medium.
Japanese Patent Application Laid-open Specification No. 1-118598 describes the use of fluorocompounds, such as a perfluoropolyether and a fluorinated silicone, as a lubricant for fluorocarbon refrigerants. However, no description is made therein with respect to the miscibility of such fluorocompounds with a hydrofluoroalkane at low temperatures below room temperature. Further, no description is made therein with respect to the miscibility of fluorocompounds, other than a perfluoropolyether and a fluorinated silicone, with a hydrofluoroalkane.
The present inventors previously examined the miscibilities of perfluoropolyethers having various structures indicated below, inclusive of perfluoropolyethers (trade name: Fomblin Y-06, Y-25 and Y-45, manufactured and sold by Nippon Montedison Co., Ltd., Japan) employed in Examples of Japanese Patent Application Laid-Open Specification Nos. 60-96684 and 1-118598, with a fluoroalkane, such as HFC-134a, HFC-134 and HFC-152a. ##STR1##
wherein each of q.sup.1, q.sup.2, q.sup.3, q.sup.4, q.sup.5 and q.sup.6 is a positive integer.
As a result, it was found that there were cases where miscibilities were observed at temperatures as high as about room temperature or more, but in any case, miscibilities were unsatisfactory at low temperatures. Accordingly, it was confirmed that such perfluoropolyethers were not suitable as a lubricant for use in refrigeration equipment employing a fluoroalkane, such as HFC-134a, HFC-134 and HFC-152a, as a refrigerant. Moreover, because such perfluoropolyethers are not miscible with various types of hydrocarbon oils, such as alkylbenzene and mineral oil, the perfluoropolyethers cannot also be used in the form of a mixed lubricant thereof with the above-mentioned hydrocarbon oils, for use in refrigeration equipment employing a hydrofluorocarbon refrigerant.
On the other hand, in EP 0 528 043 A1, it is described that the fluorine-containing aromatic compound represented by formula R(XR.sub.f).sub.n, the definition of which is substantially the same as that of formula (A) hereof, exhibits excellent miscibility with HFC refrigerants, while exhibiting desirable characteristics, such as satisfactory heat resistance, lubricating properties and durability. It is also described therein to the effect that the compound of formula R(XR.sub.f).sub.n can be used in the form of mixtures with other oils; that the other lubricant oil usable in combination with the above compound is generally selected from those miscible with a fluoroalkane refrigerant to a certain degree, and more particularly, the other lubricant oil is selected from the group consisting of perfluoropolyether oils, perfluoropolyether oils having a carbonyl-containing group, such as a carboxyl group, a carboxylate group, an amido group, a ketone group and an ester group, or having a polar group, such as a hydroxyl group, an amino group, an imido group, an ether group, a benzoimidazol group, a phosphite group, a phosphine group, a nitrile group, a phosphotriazine group and a triazine group, chlorofluorocarbon oils, polyalkylene glycol oils, ester oils and fluorinated silicone oils; and that appropriate types of oils are selected from these oils, taking into consideration the miscibility thereof with the compound R(XR.sub.f).sub.n and the viscosity and lubrication characteristics of the resultant lubricating composition. However, in EP 0 528 043 A1, there is no description with respect the miscibility of the these mixtures (mixed oils) with HFC refrigerants.
In these situations, the present inventors have made extensive and intensive studies with a view toward further improving the properties of the lubricant having the structure disclosed in EP 0 528 043 A1 in order to provide a practically further improved lubricant exhibiting not only excellent miscibility with HFC refrigerants, such as HFC-134a, and HFE refrigerants over a wide temperature range, but also satisfying all requirements of properties for use thereof as a lubricant in refrigeration equipment, that is, low temperature fluidity, low moisture absorption properties, high electrical insulation, prolonged durability, excellent lubricating properties, high safety, such as low bioaccumulation (bioconcentration), and low cost.
As a result, it has surprisingly, unexpectedly been found that when the fluorine-containing aromatic compound (A) represented by formula R(XR.sub.f).sub.n as defined herein is mixed with a specific alkyl- or alkyl derivative-substituted aromatic compound (B) as defined herein, the resultant lubricant oil composition exhibits very excellent properties, such as high miscibility with HFC and HFE refrigerants over a wide temperature range, low temperature fluidity, low moisture absorption properties, and high electrical insulation, as compared to the compound (A) alone, without impairing the inherent excellent properties of the compound (A), such as high heat resistance, high hydrolysis resistance, and low bioaccumulation (bioconcentration). In this connection, it should be noted that the alkyl- or alkyl derivative-substituted aromatic compound (B) is available at low cost, so that the mixing of compound (A) with such a cheap compound (B) leads to a cost reduction, bringing about great advantages from a practical point of view.
The present invention has been completed, based on such novel findings.